1. Field of the Invention
The present invention relates to an air cooled airfoil used in a gas turbine engine, and more specifically to the cooling air passages leading to an outer surface of the airfoil, the cooling air passages having a coating therein that melts away depending upon the temperature of the cooling air passing there through in order to open the cooling passage and allow for more cooling air flow.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Blades and vanes in gas turbine engines include cooling air passages leading to an outer surface of the airfoil that requires cooling. These cooling air passages are typically located in specific locations on the airfoil where extreme high temperatures exists during operation of the engine. Certain regions of the surface require larger amounts of cooling air than other areas that require less cooling air. When designing the size of the cooling air passages, the designer typically sizes the passages to be able to supply the amount of cooling air to cool the airfoil surface under the worst case situation of highest possible heat load. This design temperature, in all likelihood, will not be reached under normal operation of the engine. Also, the heat load varies on surfaces of the airfoil, so not every surface requires the same amount of cooling air flow. Thus, the amount of cooling air passing through the passage and onto the external surface of the airfoil is more than is needed to adequately cool that area of the airfoil. Thus, cooling air flow is wasted and overall engine performance and efficiency is reduced.
U.S. Pat. No. 6,408,610 issued to Caldwell et al on Jun. 25, 2002 shows in FIG. 1 a METHOD OF ADJUSTING GAS TURBINE COMPONENT COOLING AIR FLOW, in which an airfoil includes a plurality of cooling holes having a thermal barrier coating applied at various thicknesses in the holes to provide a desired hole diameter. Under this method, the size of the cooling air passages can be designed to provide a desired amount of cooling air flow onto the surface of the airfoil—depending upon the air pressure within the blade and around the opening of the cooling air passage—such that a desired amount of cooling can occur. However, the main difference between the Caldwell invention and the present invention is that the sizes of the cooling holes do not vary based upon the operating conditions of the engine in the region of the specific cooling air passage. Under this invention, the size of the cooling air passage may be smaller than needed, resulting in less cooling air flow than required, or larger than needed, resulting in more cooling air flow than required. Either way, the engine performance or efficiency is reduced.
U.S. Pat. No. 6,416,279 issued to Weigand et al on Jul. 9, 2002 shows in FIG. 2 a COOLED GAS TURBINE COMPONENT WITH ADJUSTABLE COOLING in which the cooling air passage includes different means to vary the amount of cooling air flow during engine operation. In one method, a restrictor having an opening of specific size is placed in the cooling air passage to regulate the cooling air flow during engine operation. In this method, the size of the restrictor cannot be changed during engine operation. In another method, a control system is used and includes a temperature sensor and a control valve, where the control valve regulates an amount of cooling air flow based upon a value from the temperature sensor. The present invention is different from the Weigand invention in that no complicated air control sensors and valves are needed, or the cooling air flow can be varied during engine operation.
U.S. Pat. No. 6,485,255 issued to Care et al on Nov. 26, 2002 shows a COOLING AIR FLOW CONTROL DEVICE FOR A GAS TURBINE ENGINE in which a single shape memory metal valve is disposed in a cooling passage upstream of the many cooling air passages that open out onto the outer surface of the airfoil. In the Care invention, the valve varies the air flow depending upon temperature, but all of the cooling air passages opening onto the airfoil surfaces are controlled by this single valve. The passages exposed to the hottest surface of the airfoil are regulated by the same valve and supply airflow as the openings exposed to the coolest airfoil surface.
While all of the above mentioned prior art inventions disclose various methods to regulate the flow of cooling air onto a surface of the airfoil, none show a method or apparatus that can vary the flow of cooling air through the individual passages based upon the heat load at that individual cooling air passage.